The present invention relates to a glow plug for preheating a precombustion chamber or a combustion chamber of a diesel engine and, more particularly, to a two-wire type glow plug having two terminals which are insulated from an engine body.
Since the start-up of diesel engines is generally very difficult at low temperatures, a glow plug is arranged in a precombustion chamber or a combustion chamber. A current flows through the glow plug to heat intake air, or the glow plug is used as an ignition source, thereby improving performance during starting. A conventional glow plug comprises a heater plug called a sheath type glow plug wherein a coil is embedded in a heat-insulating metal sheath filled with a thermally and electrically insulating powder. However, indirect heating is employed in the sheath type glow plug, and it takes a long period of time to heat the engine to a predetermined temperature. The current flows through the coil and transfers heat from the coil to the sheath through the thermally and electrically insulating powder. The heat is then radiated from the sheath to the cylinder, resulting in low heat transfer efficiency. For the cylinder to be heated to a temperature of, for example, 800.degree. C., it takes several tens of seconds.
When a current is increased in order to decrease the time required for heating, the coil may become disconnected, or the sheath may be damaged due to a high temperature. In addition, it is very complicated to embed the coil in the sheath with the thermally and electrically insulating powder.
Since the two terminals of the conventional sheath type glow plug are both electrically insulated from the engine body, the construction of the plug is complicated, and the assembly thereof becomes cumbersome, resulting in high cost. In addition, the size of the glow plug, especially, its diameter becomes large.
The two-wire type glow plug is used in an engine such as a marine engine or an engine using an AC power source when the plug cannot be grounded due to its specifications or other reasons. In this case, the sheath serving as one electrode of the coil must be insulated from the plug holder. The sheath leads from the electrode terminal block of the holder through a lead wire, thus increasing a radial size of the plug. Demand has arisen for a solution to these problems.
Another conventional ceramic heater glow plug designed so as to solve problem of the heat transfer efficiency of the conventional sheath glow plug has received a great deal of attention. In the ceramic heater type glow plug, a resistor is embedded in a ceramic material so as to constitute a heater rod which quickly heats the cylinder. However, an electrode terminal block of the heater rod and connections between an electrode terminal block of the holder and lead wires become complicated. Therefore, demand has arisen for a highly reliable compact glow plug having a simple construction, resulting in easy assembly and low cost.